not sure how you are rendering this out but to take advantage of the use background shader you use it in tandem with the IBL node for your HDRI and render with Mental ray with final gather on .
also turn off the reflectivity and the reflection limit by setting them at 0 and the specular to completely black in the Use Background shader.You may have to turn off the default light also in the render settings of MR.

Hope this makes it a bit clearer for you .